Question: The scale on a map is 5cm : 3km. If two cities are 15cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 5cm on the map corresponds to an actual distance of 3km. A distance of 15cm on the map is the same as 3 $\cdot$ 5cm. The actual distance between the two cities is 3 $\cdot$ 3km, or 9km.